I just released a new version (1.0) of my plugin Spammer Blocker. It blocks all visitors who posted a comment that was later marked as spam. It works especially efficient if you also use Akismet. I haven't received any feedback yet. Please install the plugin and let me know if there is something wrong. Thanks.
Why do you need to "block" them if you never see them? Adjusting the htaccess to block India and China will block 90% of them anyway.